The present invention pertains to an improved process for the lithographic reproduction of multicolored visual subject matter having gradations of tonal density such as photographs and paintings. More specifically, the invention provides a method for significantly improving the shadow detail, dimension and depth, as well as increasing the spectural range of multicolor lithographic reproduction. As used herein, the term "color" includes black and the term "primary color" means magenta, cyan or yellow.
In the Rapoport et al, U.S. Pat. No. 3,581,660, the applicants describe a process for the photolithographic reproduction of grey tone, i.e., black and white visual subject matter including dual printing steps, each done with a printing plate made be well known photolithographic techniques from a master and secondary negative produced by exposing the original subject matter to the light source through a patterned transparent screen. The screen used to make the master negative is a conventional halftone screen while the screen used for the secondary negative is a random pattern contact screen. The secondary negative is exposed to the original copy for a longer time interval, under the same lighting conditions, to produce a printing plate that when printed in registry with the master printing plate results in a final reproduction having extraordinary detail in the shadow areas of the original copy.
Prior art patents disclosing the use of other than halftone images and printing plates for making multicolored lithographic reproductions include among others the Cooke U.S. Pat. No. 3,130,669; the Weyl U.S. Pat. No. 809,157 and that Gast U.S. Pat. No. 547,780.
The Cooke patent describes a complex procedure for making a "screenless" halftone negative from each color separation by employing a granular surfaced screen interposed between a treated and bleached continuous tone color separation of the original subject matter and a light source during exposure. The Cooke patent teaches a method for multicolored lithographic reproduction which closely parallels the widely used four color halftone method with the modification of utilizing granular screens in place of the ruled halftone screens in making the images from which the printing plates are produced.
Similarly, the Weyl patent teaches that the "moire" effect can be eliminated in multicolored lithography by substituting a "stipple" pattern screen for the ruled halftone screen in making each of the printing plate negatives from the color separations. The Gast patent describes a three color lithographic process employing diagonally ruled halftone screens for making the blue and red printing plate negatives and a stipple screen for making the yellow printing plate negative.
High quality, multicolored photolithographic reproductions of the type currently in widespread commercial use for the printing of magazines and books, for example, are produced by the technique known in the art as four color halftone lithographic printing. Four color halftone lithographic printing involves the production of four separate halftone negatives, each from a separate continuous tone negative color separation of the original subject matter and the subsequent production of a corresponding printing plate from each of the halftone negatives. The final reproduction is obtained by printing a substrate with all four printing plates on a four color printing press in sequence and in registry, using a printing ink for each plate that approximates the color of the corresponding areas of the original subject matter. Typically, a halftone negative and corresponding printing plate is made for each of the primary colors, magenta, cyan and yellow and optionally, one for black, by techniques well known in the art.
The final prints from the standard three or four color halftone process are limited in detail in shadow or dark areas relative to the original subject matter and inherently lack the color spectrum range of the original copy since only the primary colors are printed. A further significant limitation of the conventional halftone color process is that additional strong colors such as green, orange and purple cannot be printed in addition to the four primary colors since a moire effect would almost surely result. The moire effect is well known in the art and is manifested by a mechanical appearing pattern in the final image when a number of halftone prints are made in registry as in color lithography. To avoid the moire effect in the conventional four color process, each of the halftone negatives are made at a different screen angle to prevent the respective halftone dot patterns from directly overlying each other. The conventional color lithographic process is limited in the number of strong colors that can be used by the availability of only three or four screen angles that are far enough apart, i.e., 150.degree. to 30.degree. to avoid a moire effect.
A primary objective of the invention is to provide a multicolor lithographic reproduction process wherein the accuracy of color reproduction and depth and dimensional perception, relative to conventional halftone color reproductions, is substantially improved to an unexpected degree without creating a moire pattern.
A further objective of the invention is to reproduce greater shadow detail in multi-color lithographs. A still further objective of the invention is to increase the color spectrum reproduced by color lithography. A still further objective of the invention is to provide a method for utilizing the screens and teachings of the applicants' prior U.S. Pat. No. 3,581,660 for the improved lithographic reproduction of multicolored subject matter. A still further objective is to produce multicolor lithographic prints with a minimum, if any, color correction prior to making the final negatives from which the printing plates are produced.